Question

The project I’m working currently on has a way to define a filter on objects from a database.
This filter is a pretty straightforward class containing criteria that will be combined to produce a SQL where clause.

The goal now is to use this class to filter .Net objects as well. So for example the filter might specify that the title property of the object that it is applied to must contain some user-defined string.

What are ways to approach this problem? What should the filter return instead of the sql where-clause and how can it be applied to the object? I’ve been think about this for hours and don´t yet have even a slight idea how to solve this. Been thinking about reflection, dynamic code execution, building expressions but still haven´t found a starting point.

Answer 1

It depends on what all the use-cases are 🙂

If your DB code could use LINQ, then I might consider returning an Expression<Func<YourEntity,bool>>, as this can be applied via .Where to both DB-based LINQ and LINQ-to-Objects – in the latter case simply by calling:

var filtered = original.AsQueryable().Where(filter);

(DB code is typically already queryable, so no AsQueryable is needed).

One bad thing about this is that there is no guarantee that a particular query expression will work on every LINQ provider.

Alternatively, you would have to write either your own basic query language, or a basic tree-structure. And then write code to translate to both property-reflection and SQL. Lots of work.

One final thought is to just have two different query results, and keep the two thigs separate. So one SQL, one Predicate<T> or similar. There is a risk of them being out of sync, though.